The invention relates to squeeze casting or forming a metal article from a molten charge under pressure, and more particularly to an improved squeeze casting apparatus and method wherein articles of predetermined and uniform dimensions may be produced even though quantities of poured molten casting material may vary from cycle to cycle.
Squeeze casting, or forming metal articles in a die under high pressure during solidification, has been known for some years. See, for example, U.S. Pat. Nos. 3,228,073 and 3,613,768 which show various forms and aspects of squeeze casting. Parts produceable by squeeze casting have been otherwise produced by either conventional casting or forging methods or by machining.
In squeeze casting, the molten casting material is subjected to a high pressure, usually 4000 to 20,000 p.s.i., by the action of a two-piece die mounted on a hydraulic press. A "punch" or upper die piece moves into and seals with a precisely finished female die cavity that forms the outside of the part and initially receives the charge of molten material. Upon closing, the punch die and the female die define a closed die cavity space which is completely filled with the molten material by displacement due to the continued advancement of the punch die. The desired pressure is then exerted on the punch die and on the material within the die cavity and continues while the cast article solidifies. The result is a high density, porosity free article having a fine grained microstructure and a good surface finish. The squeeze cast article has properties comparable to forgings but at casting costs. In fact, the strength properties of squeeze cast articles generally equal or exceed those of articles wrought or forged from the same alloy. The process is adaptable to both ferrous and nonferrous alloys.
There are two variations of squeeze casting. In one, solid parts are produced simply by utilizing solidification under pressure. An important consideration with this variation is the casting's height-to-diameter ratio, which should be below a certain maximum for effective squeeze casting. The other variation may be described as molten metal extrusion, whereby hollow parts are produced. As discussed above, molten metal is forced upward as the punch die descends, until the volume of the closed cavity matches that of the poured metal. At this time, full pressure is applied to the solidifying metal with a hydraulic shock. With both variants there is a beneficial fine grained microstructure obtained because of the rapid rate of solidification achieved.
Certain shortcomings of squeeze casting as heretofore known have resulted from the fact that precise control could not be maintained over the quantity of poured metal admitted to the squeeze casting die in each operation. This imprecision dictates that squeeze casting can be used only when certain dimensions of a component are not critical or when extensive machining of the cast part can be accepted. It also prevents the use of a single press head affixed to a plurality of punch dies coacting with respective female dies, since squeeze casting pressure cannot be maintained on such plural castings without exactly corresponding quantities of poured metal in each of the squeeze casting cavities. If one cavity closes on its charge of molten metal and reaches the desired pressure before the remaining cavities, it will obviously be the only cavity to reach and maintain the desired pressure, and only one properly squeeze cast article will result.
Partial solutions to these problems have been suggested, and in some cases used. When an outside height dimension on a squeeze cast component is critical, a way to circumvent the problem of charge variations has been to locate the dimension which is subject to variation on the inside surface of the component. To this end, a "telescoping" punch die may be utilized. Such a punch die includes a flange which actually comes to rest against the lower die before pressure is applied. A telescoping portion of the punch is retractable into the punch, and when the punch has come to rest against the lower die, pressure is applied by forcing the telescoping portion toward the cavity. Since charge quantities are not precisely consistent, the telescoping portion leaves a variable dimension which should be located in an area where dimensions are not critical. The remaining dimensions of the component are of course controlled by the fixed dimensions of the cavity resulting from the fact that the punch die rests against the lower die and does not itself provide casting pressure. In this sense, use of such a casting apparatus more closely approximates die casting than squeeze casting, with attendant disadvantages. Other limitations of this type casting apparatus are that it cannot be used when every dimension of a component is critical, and that the telescoping punch tends to localize casting pressure in the area of the telescoping portion as stiffening occurs.
U.S. Pat. Nos. 3,068,539, 3,120,038 and 3,387,646 show casting or molding processes somewhat similar to squeeze casting. In these patents a central die similar to a punch die is lowered into a cavity containing poured molten casting material, which rises to fill the resulting cavity as the punch die continues downward. However, as in the above described telescoping punch apparatus, casting pressure is not obtained by the force of this central die against the body of casting material, since the movable central die hits a rigid stop to limit its downward motion. During the final portion of the travel of the central die, excess casting material is forced out of a narrow, restricted annular passageway around the top of the cavity defining the article to be formed. An excess charge is thus compensated for in this way. At this point, the molten casting material has been subjected to only a low pressure. After the metal has partially solidified, pressure pins at the bottom or sides of the die cavity are forced inwardly toward the die cavity to compensate for shrinkage of the molten metal which takes place as the metal solidifies. It is stated that since the metal in the narrow, restricted overflow passage has already solidified before the pressure pins are moved inwardly, the movement of the pins does not cause further displacement of the metal through the passage.
Although the above patents disclose a means for controlling the dimensions of a cast component, the process is very different from squeeze casting and from the present invention in that pressure on the cast component is exerted only after the metal has partially solidified, and it is introduced into a localized area. Thus, the properties of the resulting cast component are generally inferior to those of squeeze casting since metal flow throughout the entire part is not as effectively controlled during solidification. Compensation for charge size variation is achieved not by the pressure pins, but by the overflow of molten material which forms a large, uneven ring of flash on the cast article. Effective, efficient dimensional control in combination with the benefits of squeeze casting are thus not provided by the apparatus and method of the above patents, as they are by the present invention described below.